11.3.9 An Easy Way to Use Library Functions

In Include Files, we saw how gawk provides a built-in
file-inclusion capability. However, this is a gawk extension.
This section provides the motivation for making file inclusion
available for standard awk, and shows how to do it using a
combination of shell and awk programming.

Using library functions in awk can be very beneficial. It
encourages code reuse and the writing of general functions. Programs are
smaller and therefore clearer.
However, using library functions is only easy when writing awk
programs; it is painful when running them, requiring multiple -f
options. If gawk is unavailable, then so too is the AWKPATH
environment variable and the ability to put awk functions into a
library directory (see Options).
It would be nice to be able to write programs in the following manner:

The following program, igawk.sh, provides this service.
It simulates gawk’s searching of the AWKPATH variable
and also allows nested includes; i.e., a file that is included
with ‘@include’ can contain further ‘@include’ statements.
igawk makes an effort to only include files once, so that nested
includes don’t accidentally include a library function twice.

igawk should behave just like gawk externally. This
means it should accept all of gawk’s command-line arguments,
including the ability to have multiple source files specified via
-f, and the ability to mix command-line and library source files.

The program is written using the POSIX Shell (sh) command
language.78 It works as follows:

Loop through the arguments, saving anything that doesn’t represent
awk source code for later, when the expanded program is run.

For any arguments that do represent awk text, put the arguments into
a shell variable that will be expanded. There are two cases:

Literal text, provided with --source or --source=. This
text is just appended directly.

Source file names, provided with -f. We use a neat trick and append
‘@include filename’ to the shell variable’s contents. Since the file-inclusion
program works the way gawk does, this gets the text
of the file included into the program at the correct point.

Run an awk program (naturally) over the shell variable’s contents to expand
‘@include’ statements. The expanded program is placed in a second
shell variable.

Run the expanded program with gawk and any other original command-line
arguments that the user supplied (such as the data file names).

This program uses shell variables extensively: for storing command-line arguments,
the text of the awk program that will expand the user’s program, for the
user’s original program, and for the expanded program. Doing so removes some
potential problems that might arise were we to use temporary files instead,
at the cost of making the script somewhat more complicated.

The initial part of the program turns on shell tracing if the first
argument is ‘debug’.

The next part loops through all the command-line arguments.
There are several cases of interest:

--

This ends the arguments to igawk. Anything else should be passed on
to the user’s awk program without being evaluated.

-W

This indicates that the next option is specific to gawk. To make
argument processing easier, the -W is appended to the front of the
remaining arguments and the loop continues. (This is an sh
programming trick. Don’t worry about it if you are not familiar with
sh.)

-v, -F

These are saved and passed on to gawk.

-f, --file, --file=, -Wfile=

The file name is appended to the shell variable program with an
‘@include’ statement.
The expr utility is used to remove the leading option part of the
argument (e.g., ‘--file=’).
(Typical sh usage would be to use the echo and sed
utilities to do this work. Unfortunately, some versions of echo evaluate
escape sequences in their arguments, possibly mangling the program text.
Using expr avoids this problem.)

--source, --source=, -Wsource=

The source text is appended to program.

--version, -Wversion

igawk prints its version number, runs ‘gawk --version’
to get the gawk version information, and then exits.

If none of the -f, --file, -Wfile, --source,
or -Wsource arguments are supplied, then the first nonoption argument
should be the awk program. If there are no command-line
arguments left, igawk prints an error message and exits.
Otherwise, the first argument is appended to program.
In any case, after the arguments have been processed,
program contains the complete text of the original awk
program.

The awk program to process ‘@include’ directives
is stored in the shell variable expand_prog. Doing this keeps
the shell script readable. The awk program
reads through the user’s program, one line at a time, using getline
(see Getline). The input
file names and ‘@include’ statements are managed using a stack.
As each ‘@include’ is encountered, the current file name is
“pushed” onto the stack and the file named in the ‘@include’
directive becomes the current file name. As each file is finished,
the stack is “popped,” and the previous input file becomes the current
input file again. The process is started by making the original file
the first one on the stack.

The pathto() function does the work of finding the full path to
a file. It simulates gawk’s behavior when searching the
AWKPATH environment variable
(see AWKPATH Variable).
If a file name has a ‘/’ in it, no path search is done.
Similarly, if the file name is "-", then that string is
used as-is. Otherwise,
the file name is concatenated with the name of each directory in
the path, and an attempt is made to open the generated file name.
The only way to test if a file can be read in awk is to go
ahead and try to read it with getline; this is what pathto()
does.79 If the file can be read, it is closed and the file name
is returned:

The main program is contained inside one BEGIN rule. The first thing it
does is set up the pathlist array that pathto() uses. After
splitting the path on ‘:’, null elements are replaced with ".",
which represents the current directory:

The stack is initialized with ARGV[1], which will be ‘/dev/stdin’.
The main loop comes next. Input lines are read in succession. Lines that
do not start with ‘@include’ are printed verbatim.
If the line does start with ‘@include’, the file name is in $2.
pathto() is called to generate the full path. If it cannot, then the program
prints an error message and continues.

The next thing to check is if the file is included already. The
processed array is indexed by the full file name of each included
file and it tracks this information for us. If the file is
seen again, a warning message is printed. Otherwise, the new file name is
pushed onto the stack and processing continues.

Finally, when getline encounters the end of the input file, the file
is closed and the stack is popped. When stackptr is less than zero,
the program is done:

The shell construct ‘command << marker’ is called a here document.
Everything in the shell script up to the marker is fed to command as input.
The shell processes the contents of the here document for variable and command substitution
(and possibly other things as well, depending upon the shell).

The shell construct ‘$(…)’ is called command substitution.
The output of the command inside the parentheses is substituted
into the command line.
Because the result is used in a variable assignment,
it is saved as a single string, even if the results contain whitespace.

The expanded program is saved in the variable processed_program.
It’s done in these steps:

Run gawk with the ‘@include’-processing program (the
value of the expand_prog shell variable) on standard input.

Standard input is the contents of the user’s program, from the shell variable program.
Its contents are fed to gawk via a here document.

The results of this processing are saved in the shell variable processed_program by using command substitution.

The last step is to call gawk with the expanded program,
along with the original
options and command-line arguments that the user supplied.

eval gawk $opts -- '"$processed_program"' '"$@"'

The eval command is a shell construct that reruns the shell’s parsing
process. This keeps things properly quoted.

This version of igawk represents my fifth version of this program.
There are four key simplifications that make the program work better:

Using ‘@include’ even for the files named with -f makes building
the initial collected awk program much simpler; all the
‘@include’ processing can be done once.

Not trying to save the line read with getline
in the pathto() function when testing for the
file’s accessibility for use with the main program simplifies things
considerably.

Using a getline loop in the BEGIN rule does it all in one
place. It is not necessary to call out to a separate loop for processing
nested ‘@include’ statements.

Instead of saving the expanded program in a temporary file, putting it in a shell variable
avoids some potential security problems.
This has the disadvantage that the script relies upon more features
of the sh language, making it harder to follow for those who
aren’t familiar with sh.

Also, this program illustrates that it is often worthwhile to combine
sh and awk programming together. You can usually
accomplish quite a lot, without having to resort to low-level programming
in C or C++, and it is frequently easier to do certain kinds of string
and argument manipulation using the shell than it is in awk.

Finally, igawk shows that it is not always necessary to add new
features to a program; they can often be layered on top.

As an additional example of this, consider the idea of having two
files in a directory in the search path:

default.awk

This file contains a set of default library functions, such
as getopt() and assert().

site.awk

This file contains library functions that are specific to a site or
installation; i.e., locally developed functions.
Having a separate file allows default.awk to change with
new gawk releases, without requiring the system administrator to
update it each time by adding the local functions.

One user
suggested that gawk be modified to automatically read these files
upon startup. Instead, it would be very simple to modify igawk
to do this. Since igawk can process nested ‘@include’
directives, default.awk could simply contain ‘@include’
statements for the desired library functions.